The present invention relates to a method of recovering magnetic powder from rare earth bond magnets and a method of recycling the magnetic powder.
Rare earth bond magnets formed from a molding compound containing magnetic powder and binders are used in motors, plastic sheets, magnet rollers, magnet sensors or medical devices.
Generally, most magnetic materials are hard and brittle. In the case of cast magnets and sintered magnets, magnetic materials undergo casting, sintering and heating, and grinding to predetermined dimensions. Therefore, it is hard to produce either thin products, or complex-shaped products. Furthermore, problematic cracks or chips occur. To compensate for these disadvantages, bond magnets are used.
Crack resistant, lightweight bond magnets have dimensions of high accuracy, and can be manufactured readily. But, bond magnets have inferior magnetic properties compared to cast magnets or sintered magnets because bond magnets contain binders about 10 to 50 vol %. Recently, as strong magnets like rare earth bond magnet have been provided, production of bond magnets is rapidly increasing. Both thermosetting resin and thermoplastic resin are used for binders. Thermosetting resin is generally used for producing a bond magnet by compression molding. Thermoplastic resin is used for producing a bond magnet by extrusion molding. In a bond magnet produced by compression molding, large amounts of magnetic powder are packed as compared to injection molding. Consequently, the product has improved magnetic properties as compared with the product produced by injection molding.
Japanese Patent Laid Open Publication No. 59-136907 discloses recycling rare earth bond magnet molded by molding compounds containing magnetic powder and binder. Specifically, this reference discloses;
1. When R2TM17 (wherein R designates rare earth elements like samarium and TM designates transition metals like cobalt) is used as Smxe2x80x94Co magnet based injection molded bond magnets containing thermoplastic resin, sprue and runner can be pulverized and can be recycled in the injection molding again. It is difficult in the case with RCo5.
2. Demagnetizing injection-molding magnet, sprue or runner is advantageous to work.
3. Mixing materials recovered from bond magnets, sprue or runner with raw materials kneaded prevents degradation of magnetic properties and mechanical properties of the bond magnets.
The above prior art describes the efficient manufacturing of Smxe2x80x94Co based bond magnets by injection molding on an industrial scale.
Japanese Patent Laid Open Publication No. 10-55908 discloses a recycling method of magnetic powder by heating bond magnet and decomposing resin.This method expends a great amount of energy as recovering magnetic powder is done at a temperature as high as 500-1200xc2x0 C. Further, it is desirable to recover magnetic powder in an inert gas atmosphere like nitrogen, argon, or helium to prevent magnetic powder from oxidizing. Magnetic properties may deteriorate by heating at high temperatures. Moreover, the block shaped recovered magnetic powder needs to be pulverized and classified. This method is also used for recovering bond magnets with thermoplastic resins produced by injection molding or extrusion molding.
However, in a small, injection or extrusion molded magnet of not more than several grams, sprue and runner occupy 80-90% of the weight of materials used, and thus the concentration of sprue and runner is more than the produced magnet in weight. Consequently, both the energy expended on fabrication, and the rate of material deterioration increases. These results are less preferable from the standpoint of saving energy and resources.
Furthermore, it is necessary to adjust volume fraction of magnetic powder generally at not higher than 65 vol % to injection-fill kneaded-plastic fused strand into the cavity of a mold. This conflicts with the objective of increasing the volume fraction of magnetic powder, whereby more rare earth magnetic powder maximizes magnetic properties.
To increase the volume fraction of rare earth magnetic powder, thereby utilizing its original magnetic properties, it is advantageous to strongly compress rare earth magnetic powder with thermosetting resin, for example epoxy resin. In compression molding, neither sprue nor runner are produced, thus improving magnetic properties.
A compression-molded rare earth bond magnet with neodymium based rare earth magnetic powder usually contains an epoxy resin binder, and is widely used for the spindle motors of HDD""s, CD-ROM""s and/or DVD""s . Epoxy resin is generally coated on the neodymium-based bond magnet by spray coating or electro-deposition to prevent rusting.
As rare earth bond magnets with thermosetting resin cannot be refused, the prior art method for recovering and recycling bond magnets with thermoplastic resin cannot be applied to bond magnets with thermosetting resin. Therefore, the art on recycling of rare earth bond magnets by compression-molding a molding compound containing rare earth magnetic powder and thermosetting resin like epoxy resin has not been disclosed.
Rare earth elements in rare earth bond magnets are expensive. It is desirable to recover and recycle them for effective resource usage. However, as mentioned above, only bond magnets using thermoplastics are concerned in the prior art recovering technology. In the prior art recovering technology, pulverizing and heating treatment are necessary, thus possibly deteriorating the magnetic properties of the magnetic powder. Therefore, the prior art is deficient in recovering.
For recycling magnetic powder in the bond magnet with thermosetting resin, resin powder and magnetic powder are separated by fine pulverizing and specific-gravity separating. However, particle diameter of magnetic powder becomes small, and fine particle powders increase by pulverizing. Thus, properties of magnetic powder are destroyed or decreased. Further, it is hard to separate completely resin powder and magnetic powder and to recover 100% of magnetic powder.
The purpose of the present invention is to provide a method of recovering magnetic powder readily with little deterioration of magnetic properties and to provide a method of recycling the recovered magnetic powder for both of rare earth bond magnets using thermoplastic resin and thermosetting resin as the binder.
The present invention provides a recovering method of magnetic powder comprising the steps of soaking rare earth bond magnet in a decomposing solution containing at least one solvent selected from the group consisting of tetralin, naphthalene, methylnaphthalene, 1,4-hydroxylnaphthalene, naphtol, biphenyl, 2-hexanone, acetonylacetone, phorone, cyclohexanone and methylcyclohexanone and then heating at a temperatures of 230xc2x0 C. or higher. The other process comprises the step of holding rare earth bond magnets in the gas phase of the decomposing solution and then heating at a temperatures of 230xc2x0 C. or higher. The present invention also provides a recycling (or re-use) method of recovered magnetic powder, which is recycled in at least a part or all of magnetic powder contained in molding compound for molding rare earth bond magnet.
For preventing deterioration by surface oxidation of magnetic powder, it is preferable to substitute the air in the vessel with at least one of nitrogen, helium and argon , or to evacuate inside of the vessel to 10xe2x88x922 Torr or lower after putting rare earth bond magnet into a decomposition vessel. Adding antioxidant or reducing agent into the decomposing solution is preferable to enhance prevention of deterioration by surface oxidation of magnetic powder.
To maintain the magnetic properties of bond magnets, the amount of recovered magnetic powder substituting the magnetic powder in the molding compound should be preferably 20 wt % or less.